The present invention relates to methods and apparatus for use in the production of a gas stream containing a product gas from a reactant gas, e.g. the production of an ozone containing gas stream from oxygen.
Ozone is produced industrially for use for a number of purposes. These include disinfection, e.g. of drinking water, and a number of chemical reactions. As ozone is unstable and decays back to oxygen, it must be made immediately prior to its use. This is conventionally done in an ozoniser in which either air or oxygen is subjected to a silent electric discharge. It is known that the efficiency of the ozone generation depends upon the oxygen concentration in the gas subjected to this process and rises with increasing oxygen concentration. At higher ozone concentrations, the specific power required for ozone generation from oxygen may be lower than 10 percent of that required using air.
However, a disadvantage of the use of oxygen is the cost of separating oxygen from air. Only a low overall conversion of oxygen to ozone is achieved in the ozoniser, typically from 5 to 15 wt % so that large amounts of oxygen will be wasted if there is no use for the oxygen enriched gas downstream of the ozone contacting process in which the ozone is used. Various attempts have been made in the past to limit this oxygen wastage.
A technique known as long-loop recycling is well known in the literature and is described in the "Handbook of Ozone Technology and Applications" Vol. 2 (Ed. Rip G. Rice--An. Abor. Science). In this technique, an oxygen-rich gas supplied either from the liquid oxygen tank or as the output of an air separation unit is subjected to the production of ozone in an ozoniser. The oxygen-rich ozone containing gas is then used in the ozone consuming process such as water treatment. Residual ozone in the off-gas from this process is then destroyed leaving an oxygen-rich vent gas contaminated by chemical species picked up in the ozone consuming process. These will typically include water and nitrogen as well as hydrocarbons but will depend upon the nature of the ozone consuming process. These contaminants must be removed from the oxygen-rich stream in a drying and clean-up stage prior to the oxygen being recycled into the system upstream of the ozoniser or they will upset the operation of the ozoniser.
This process is disadvantageous in that the purification of the oxygen-rich vent stream poses a complex problem. The nature of amount of all of the different contaminants which may be present is not necessarily known and, if known, is likely to be unique to the particular process conditions used so that the drying and clean-up stage must be customised to deal with each unique situation. There is a continuing risk that the recycled oxygen-rich stream may carry moisture into the ozone generator and this will cause power inefficiencies in the generator and eventual permanent damage. Where the process is one of water treatment, the ozone contacting process will result in large amounts of nitrogen being stripped out of the water stream by the oxygen/ozone mixture which will increase the load on the recycling system which must remove hydrocarbons, water and nitrogen and other inert gases to bring the oxygen concentration to feed gas purity. Failure to achieve this level of purity will affect the power consumption of the ozone generator.
Although the bulk of the oxygen can be recovered, there are significant oxygen losses in the clean-up and drying processes.
An alternative recycling process known as short-loop recycling is described in U.S. Pat. No. 2,872,397 (Kiffer) and with variations in numerous other publications. In this type of system oxygen is supplied as before to an ozoniser to form an oxygen-rich ozone containing stream. Before the ozone consuming process however the oxygen is separated from this stream and recycled back upstream of the ozoniser. The ozone is transferred to another gas stream such as air. The ozone containing gas is then led to the ozone consuming process. Off-gas from that is subjected to the destruction of residual ozone and may then be vented.
The mechanism for separating the oxygen from the ozone is typically the use of a PSA (pressure swing adsorbent) system in which the oxygen/ozone mixture is passed through a solid adsorbent which retains the ozone and passes the oxygen. When the adsorbent bed becomes sufficiently saturated, a dry inert gas, typically a nitrogen-rich stream from the air separation unit, is passed counter-current to the oxygen flow through the bed to desorb the ozone from the adsorbent and to produce a nitrogen-rich ozone containing stream which is fed to the ozone consuming process. An oxygen-rich stream is then used to displace the nitrogen from the bed before the bed goes back on-line to adsorb ozone. The use of three beds allows these steps to be conducted cyclicly on each bed with one bed being on-stream at all times.
In U.S. Pat. No. 2,872,397, at the end of the on-line period oxygen is exhausted from the void volume of the bed by using a vacuum system to reduce the pressure in the bed and this evacuated low pressure gas is recovered and recompressed for recycling into the system. This of course involves a power consumption cost. If it is not done, all of the oxygen co-absorbed with the ozone in the bed and the void volume of oxygen in the bed is lost from the system when the cycle is switched to the desorption of ozone from the bed using a nitrogen-rich stream. This switch loss of oxygen significantly reduces the oxygen recovery in the cycle. It also passes oxygen into the ozone consuming process which may or may not be acceptable depending upon the nature of that process. A similar system is described in U.S. Pat. No. 4,136,027 (Sakamoto et al). The process uses adsorbent preferably silica gel operating preferably at low temperature. The process needs a source of refrigeration and a heat exchange system for cooling feed against product streams. Apart from this complexity the process suffers from potential safety problems because the high adsorption capacity of silica gel for ozone at low temperature. Maloperation can cause desorbtion of ozone at concentrations high enough for explosions to occur.
U.S. Pat. No. 4,371,380 (Benkmann) illustrates the use of this type of system in a context other than ozone generation. Here, oxygen is passed from an air separation unit to a fermenter in which it becomes loaded with CO.sub.2 which needs to be removed so that the oxygen can be recycled into the fermenter. This is carried out in a PSA system in which the carbon dioxide is adsorbed and periodically removed from the adsorbent using a flow of nitrogen from the air separation unit.
In U.S. Pat. No. 4,280,824 (Lassmann et al) the nitrogen-rich gas used for desorbing the ozone is air and the adsorbent contains activated alumina for adsorbing water vapour from the air, silica gel for adsorbing the ozone as well as carbon dioxide contained in the air and molecular sieve for adsorbing nitrogen. Accordingly, this system combines the separation of air into nitrogen and oxygen with the recycling of the oxygen using the same adsorbent beds.
Whereas in U.S. Pat. No. 2,872,397, the adsorbent was cleared of nitrogen by introducing oxygen counter-current to the normal adsorption flow direction, in U.S. Pat. No. 4,786,489 (Grenier) the oxygen is used to flush nitrogen out of the adsorbent beds in the co-current direction with respect to the adsorption flow.
JP-63-159202 (Mitsubishi Heavy Industries) contains a disclosure similar to that of U.S. Pat. No. 2,872,397 but operates the ozone recovery in a refrigerated space.
The disclosure of U.S. Pat. No. 4,863,497 is similar to that of U.S. Pat. No. 4,786,489 except that it also discloses a radial flow adsorbent bed apparatus for conducting the method.
U.S. Pat. No. 5,507,957 (Garrett et al) discloses a method of this general type in which the ozonised oxygen is directed back to the PSA air separation unit in which the oxygen was produced and the ozone is adsorbed in a bed of silica gel from which it is desorbed by nitrogen flowing out of the air separation unit. This is similar to the method employed in U.S. Pat. No. 4,280,824.
There are numerous other prior art teachings essentially cumulative with those discussed above.
All of these short-loop recycling methods suffer from the disadvantages discussed at the outset.